Officially, MJOLNIR MK. IV was supposed to look like MJOLNIR MK. V with a larger fusion pack. However, as armor customization was introduced into Halo's multiplayer, it was mad that the large undertaking that Project MJOLNIR was included multiple iterations and variants of each version. So it can technically be explained that the MJOLNIR MK. IV we see in Forward Unto Dawn is actually a variant of MJOLNIR. It can also be chalked up to artistic licence.